One of the most memorable episodes of "Growing Pains" for me was "Reputation," in which Mike intends to cheat on a test by writing the answers on his sneakers, but in the process learns the material. After the teacher sees the writing on his sneakers and accuses Mike of cheating, Mike's parents convince the teacher to let Mike retake the test...in his boxers and undershirt. Getting to see Mike half-naked was a treat for me at the time (I was 14 and he was 15), but I also remember that he had really dirty feet--as I recall the soles were practically black. (You can watch this episode online, which I just did. He's got a nice bulge in his jeans.)
In the movie Listen to Me, which someone already mentioned, there's a scene in which he is barefoot, and again, his feet were really dirty.